Alloys and rectifiers made thereof



June 17, 1952 K. LARK-HoRovlTz ET AL 2,600,997

ALLOYS AND RECTIFIERS MADE THEREOF 2 SHEETS-SHEET l Original Filed July l5, 1945 June 17, 1952 K. LARK-HoRovlTz ET AL 2,600,997

ALLOYS AND RECTIFIERS MADE THEREOF 2 SHEETS-SHEET 2 Original Filed July 13, 1945 Patented June 17, 1952 2,600,997 ALLoYs YAND RECTIFIERS MADE 'rHEREor Karl Lark-Horovitz and Randall M. Whaley, La Fayette, Ind., assignors to Purdue Research Foundation, L'a Fayette, Ind., a corporation of Indiana Original application .luly 13, 1945, Serial No. 604,744, n oyv lIjaimnj; No. 2,514,879, dated July 11, 1950. Divided and this application December 29, 1949,-Se'rial No. 135,747

This application is a division ci application,

Serial No. 604,744, filed July 13, 1945, n ow Patent No. 2,514,879.

The present invention relates to an improve- `14 Claims. (Cl. F75-366) 3. Requirement of power for heating a cathode.

4. Require a large amount of space as compared to a contact rectifier.

The germanium alloys of our present invenment in alloys of germanium, and more par- 5 tion may be used as the semi-conductors for ticu'larly to rectiers of electricity, which offer rectiiiers of the contact type, which, accordlow resistance to current flow in one direction ing to one embodiment of our invention, possess therethrough and high resistance to current the following general advantages over known flow `in the `opposite direction, made of such alcontact rectiers:

loys. 10 1. Ability to withstand continuous operating f 11n the cllletailed description of o ioir inventor voltages greater than 10 volts in the back direco owing ,ereinaften it will be o served 't a tion, and some of which are capable of withsever'al 'of the elements which may be combined standing voltages in the back direction of an with germanium are not metals so that the reorder approaching 200 volts.

sultant materials are not alloys in 'the common 15 2. Low forward resistances, for example, 30 meaning of the word. However, `for purposes to 100 ohms at one Volt.

of the present disclosure, it is to be understood 3. High back resistances, at about 4 volts that the words alloy of germanium as used ranging from about 10,006 ohms to several herein, means to include a union of two or more megohms.

elements, one of which is germanium, and the 20 4. May be used with frequencies up to 60 other or others being metals, non-metals, or megacycles and will still rectify at 3,000 megagases, and the combination of which exhibits cycles.

electrical properties such as are found in metals 5. Provide rectiers 'of low capacity of about and semi-conductors. 0.5 micromicrofarad.

The known contact recti'ers, i. e rectifiers 25 6. Less than 50% decrease in peak back voltcomprisi'rg suitlalble file tal electrcd, a age when ambient ltemperature increases from semlcon uc or ave a eas vone o e 0 OW- 23 C. to 75 C. ing disadvJ'GPLES c 7. Do not require power for heating a cath` 1. Inability to `withstand in continuous use 0de; and v oltager thehback 01.' high lesstflcediree" 30 8. Do` not require more space than about that 131.9% `v greefte' than belli? 10W/0h75 Wlthout per needed for a common one-half watt carbon remanent lnJury to the rectler. sisi-,01.*

2- Inablllt'y 1 30 Das? Summen/5 Current? 1 n. the The germanium alloys herein disclosed are forward direction for satisfactory operat1on of au of the class OfN type Semgconductom e asoclated apamtp' i 1 b t. f th 30 semi-conductors which when made into contact t'ow. half. hrnsls al 'pmu 1. ngts .o e type rectiers present a high resistance to 'curlts elrnabgut rlo'rmcrcm s a 1s CH' rent flow across the rectifying contact when the a gquencles greater than about 1 to 5 mega' 40 lower resistance when the potential is reversed.l

5. Capacity too high to allow enicient operat. The .Vanous gelxamun aug-ys ofdourmgm' tion at frequencies greater than about 5 megatOIh/lgrgelfgrfohr aehmvl .I-Icr i ccles. f

yDue to the aforesaid deficiencies of these 45 @Ontactypeleeelfrel's- Speel electrlcell Prop" known contact reotiners, the art turned to wid'eereeehereefter referred te eref spread use of vacuum tube diodes for rectifying Peak M ck voltage-*The Yoltage-'urrent alternating currents. However, vacuum tube Chlarafe'erlstlc meallred 1:1311 ree'lletrs Singkthe diodes, while overcoming certain of the aforeal CYS 0 0\1r 1nVen 1011 S 0W a Y0 age Pea. .m mentioned disadvantages of the known contact 50 the back 0r hleh resistance dnectlon T1115 peak u n the followin disadvan# generally Occurs within a range greater than rectiers, 1n t r have g tages; y l0 volts and approaching the order of 200 v olts.

1. Inter-electrode capacities which are seri- It Will alS'O appear that` 2J1 1 0f these I'eClerS ously objectionable at high frequencies, using alloys of our invention exhibit a nega- 2, Low forward direction conductance. 55 tive resistance region in the back drectOn for currents exceeding the current at the peak back voltage.

Back resistance.-In the back or high resistance direction these rectiers have resistances ranging from the order of 10,000 ohms to several megohms as measured at about 5 volts. High resistances are substantially maintained nearly to the peak back voltage.

Forward conductance.-The currents passed at one volt in the forward or low resistance direction for these rectiers generally lie within the range between 5 milliamperes and 40 milliamperes. Actually, somewhat higher currents may be permitted to pass in the forward direction without impairment of the rectifying contact. As will be described later herein, currents greater than 100 milliamperes are sometimes deliberately passed momentarily in the forward direction to produce improvement in certain contact characteristics.

The N-type semi-conductors of our invention comprise germanium having small amounts of one of the following elements or certain combinations thereof alloyed therewith:

Copper and silver of Column I of the periodic table; Magnesium, calcium, Zinc, strontium, cadmium, or barium of Column II of the periodic table; Titanium, tin, or lead, of Column IV of the periodic table;

Nitrogen, vanadium, columbium, tantalum, or

bismuth of Column V of the periodic table; Chromium or uranium of Column VI of the periodic table. Cobalt, nickel, or palladium of Column VIII of the periodic table.

all N-type semi-conductors consisting of an alloy of germanium, as for example, the group last referred to.

Other features and advantages of our invention will appear from the detail description.

Now, in order to acquaint those skilled in the art with the manner of making alloys in accordance with our invention, and the utilization thereof as rectifiers of electricity, we shall describe in connection with the accompanying drawings and the tables following hereafter certain of the processes used in making the alloys which lie within our invention.

In the drawings:

Figure 1 shows the voltage-current characteristic curves of several rectiers using certain of the alloys of our invention, which curves are not to be taken as typical of given alloys but merely to represent the type of characteristic exhibited by such alloys in general.

Figure 2 is a graph illustrating the electrical characteristics of rectiers using different types of surfaces on one alloy of our invention.

Figure 3 is a sectional view of a rectifier, the semi-conductor of which comprises an alloy of our present invention.

Each alloy represented by the curves of Figure 1 is designated by a code number. The latter part of each code denotes the amount in atomic percent of the particular element or elements added to germanium to produce that alloy. No atomic percentage figures for the addition of nitrogen to germanium are given since it is difficult to determine accurately the amount or number of nitrogen atoms alloyed with the germanium.

In the following Table I there are set forth minimum, average, and maximum values of peak back voltage and forward current obtained on rectifying contacts using certain germanium alloys which we have made in accordance with the general procedure to be described later. The amount of the added element alloyed with germanium is set forth for each melt in atomic percent, i. e., the proportionate number of atoms in percent of the elements added to the total number of the atoms of germanium and added elements present. For purposes of adequately setting forth and claiming our invention, these additions to germanium are to be understood as being included in the term Group A used hereinafter. Substantially all melts in which the addition consisted of a single element made to date in accordance with our invention are contained in Table I. It will be observed from that table that a large number of melts with certain added elements were prepared and it will be understood that the results given are the average results of all of the melts in each instance. It is to be understood, however, that the spread or range of values given in connection with each of the elements added to germanium might not be true for any particular melt of such addition agent. Characteristics for rectifying contacts on any given alloy will lie somewhere within the range given. Further, all points on any given alloy listed in Table I and Table II, referred to hereinafter, will not exhibit the same electrical characteristics. Points may be found on each of the alloys disclosed at which the peak back voltages, back resistances, or forward currents lie in the lower regions of the ranges given above for these values. Also on the same surface of each alloy other points of contact may usually be found with electrical characteristics which lie toward the upper limit of the ranges above set out. However, as will later be discussed in more detail, some of the alloys are of greater uniformity than others with respect to rectification characteristics.

TABLE I Additions to germanium [In atomic per cent] Forward Current PelleBzsgolt at one volt D. C. Addition and percentages (Mllhampems) Min. Ave. Max. Min. Ave. Max.

.37 15 40 P'5 1 5 Pbo, .30, .50, .13, 1.08, 40 25 70 135 1 15 25 Mg: 3.0, 3.0 l0 60 115 2 10 Y 20 accuser Forward Current Peak B ack VOM' at one volt D. G. age (Volts) (Mini m Addition and'percentages a peres Min. Ave. Max. Min. Ave. Max.

1.0,`1.0, 1.0 20 50 00 7 15 Nr: Solidi'cd in N z at pressures of. 2; 18, 600, and 760, mm. Hg 2O 80 160 l 10 25 30 65 110 5 15 25 25 40 8() 7 10 2.0 .80, .50, .50, 1.0, 1.0, 1.0, 1.0, 50 v50, .50, .50,50 25 75 150 5 10 25 20 40 70 3 15 .50 .25,

25 75 150 2 15 30 30 70 3 7 l5 20 25 50 2 5 2O 10 25 55 10 25 4i) 2 5 50 100 5 l2 20 In Table `II .below there are setforth the melts in which two elements have been alloyed with germanium. `The additions of these combinations O f elements are also set forth in atomic percent as previously dened. It will be understood that ,the alloys Aset forth in this table are also to be lincluded in the .term Group A'above referred to for purposes of claiming our present invention. The peak backvoltages and the for-V ward currentsat one volt of 'rectiers made `of these alloys Vare also set forth in this table.

TABLE II Melis of more than one addition to germanium [In atomic per cent] maar tratre age o ts Addition and percentages (Mmamperes) Ave. Max. Min: Ave. Max.

.20 B,`.20 20 25 50 8 15 25 .35.CL1, 70 25 40 150 15 20 30 .21 Cu, .66 15 50 80 15 22 50 .21 011,'.98 15 v 40 65 15 30 60 .80 Ca, '.30 50 70 110 5 14 20 .80 St, .30511. 50 70 100 6 15 35 .50 Ni,.50 55 75 100 9 30 30 .50 Pd, `28() 35 `65 90 8 15 25 .50!Pd, .80 35 50 95 5 12 -20 .50 Ni, .10 39 50 95 8 15 25 .50 N, 30 20 50 75 12 20 40 .50 Ni, .30 25 45 75 10 20 35 .50 Ni, .50 30 40 50 9 12 14 .50 Ni, .50 25 35 10 15 20 .50 Ni, 1.00` 6() 90 160 10 15 25 1.00 Caf, 1. 10 30 50 13 20 25 .so Mg,'. 50 10 3o 45 10 o 20 4c 1.00 NL. .50.Sr 30 65 100 7 l() 18 1.20 S11, .32 Sr.-. 100 165 6 13 19 1.20SI1, .32 Sr 15 90 100 6 10 15 1.00 Ni. .50,S11.- 30 70 110 15 20 25 1.00-Ni, .50,SI1.- 15 50 90 1 9 19 .80 Ca, .40 Cll 35 75 140 2 7 9 80 Ca, .40 C- 15 40 75 3 5 7 10 SI1,`.l0.C`l 70 100 175 8 15 24 Ihe germanium alloys of our invention may be prepared in all cases except for the germaniumnitrogen alloy, by melting pure germanium with the desired alloying element or combination of elements in eithera high vacuum ofthe order of melting, the furnace itself, or some material volatilized in the furnace. Alloying germanium with nitrogen may be e'ected by melting the germanium in an atmosphere of nitrogen which may bepeither purified nitrogen or nitrogen direct from a commercial cylinder. The germanium is melted in nitrogen at pressures ranging from about 2mm. to 760 mm. Hg at a temperature of 10,00" to 1050 C.` Good results appear tobe independent of pressure and melts prepared within the above range of pressures were all satisfactory.

The germanium successfully used for these alloys had `purity approaching and electrical resistivity greater than about one V`chron cm. VThe germanium which we have successfully alloyed with other elements to form `thealloys listed in Tables I and II was prepared from GeOzolotained fromthe Eagle-Eicher Lead I(Iom-A pany of Joplin, Missouri. The oxide was reduced in an, atmosphere of commercial hydrogen at temperatures of 650 to '700 C. over a period of three to four hours. The oxide reduced inthis manner leaves the germanium metal in the form cfa gray-green powder which is then alloyed with anotherelement or elements in the mannerY and proportions described.

The aforesaid melts of germanium and the added element or elements were held inthe molten state long enough to allow mixing of the constituents, and it has been found that about 5 to 15. minutes is sufficient for this purpose. Usually ingredients to form melts of about five to six grams each were used in proportions above set forth in detail. After the constitutents had been allowed to mix, the melts were allowed to solidify and cool which was accomplished either by immediately removing heat or by controlled cooling apparatus. In certain cases the uniformity of the melt is aiiected bythe manner in which it is cooled.l These variations will beV discussedV later. I

A specific melt in accordance with our invention was prepared as follows:

Pure GeOz was vreduced in hydrogen at atmospheric pressure for about three hours at 650 to '700 C. Six grams of pure germanium powder so obtained were then placed in a porcelain Crucible together with small vflakes ofpure tin amounting to 25 milligrams or about 0.8Hatomi-c percent of tin.

The crucible and contents were then placed inside a graphite cylinder used as a heater inv the high frequency eld of an induction furnace, and lowered in a vertical quartz tube which was then evacuated and maintained at a pressure of about 10-5 mm. mercury. Power was then applied to the external coil of the induction furnace to melt the germanium and hold it molten for about 5 minutes. The melt was then allowed'to cool by merely turning oif the power to the coil. Thereafter wafers werecut from the alloy, and were soldered with soft-solder to a suitabl'emetal electrode to produce a very low resistance nonrectifying contact with one face of the wafer. The exposed face wasthen ground with 600 mesh alumnia and etched for 2 minutes with an etching solution consisting essentially of HNOa, HF, Cu NO3 2 and water in proportions'to be later describedherein. These wafers were then as` sembled in suitable cartridges each provided with ac'onventional metal electrode or Whisker Which was used to contact the alloy surface. Across the rectifying contact thus produced we obtain the electrical characteristics described above. a

As lmentioned in the above specic example, the surfaces of these alloys are usually ground flat and then etched in a manner to be described in detail. However, as hereinafter related, 'the etching of the alloy Ysurfaces is not essential since, vfor example, by breaking open a melt, points may be found which exhibit the aforementioned electrical rectifying characteristics. Such broken surfaces present geometrically irregular faces which introduce some diculty in assembly of the rectiers. Thus, grinding'the alloy surface at and etching it appears to be the most feasible mannerof producing the rectifiers in the commercial practicing of our invention.

From the above Table I it will be observed that the majority of experimental work conducted in the development of our invention has been with the alloy germanium-tin. Inhconnection with our experimental work with tin it has been found that above 0.1 atomic percent of tin content, the amount of tin added is not critical. GermaniumV containing above about 0.1 percent tin usually shows tin separated out, both at internal grain boundaries l-and on the outer surfaces. In some melts containing tin lin excess of 0.1 atomic percent, ductile layers of this tinrich material were frequently observed, particularly in the lower regions of the melt. In this connection we wish to observe that in making the germanium-tin alloys it is desirable in producing the melt that the boat or Crucible in which the elements are contained be gradually removed from the hot furnace region. This will produce more uniform alloys, particularly if the melt is so removed that the top region of the melt is the last part to cool. It appears that germanium becomes saturated at about 0.1 percent tin under the melting and cooling conditions used. However, in our experimental work larger amounts of tin were added in order to observe if such solubility depended upon the amount of tin available; more tin merely segregated. At 17 atomic percent addition of tin, the entire melt was interlaced with tin-rich veins which had metallic low resistance ohmic conductivity.

With bismuth additions it is difficult to control the amount of bismuth actually remaining in the germanium during the melting cycle. A considerable fraction of the bismuth volatizes so that quantities added have little relation to the quantities actually remaining in the melt. However, the results indicated in Table I in connection with bismuth were obtained by the addition of bismuth to the extent there indicated.

After the melts have been made as above described they are suitable for use as rectiers of electricity by simply making contact with the surfaces of such alloys with suitable electrodes or whiskers. In most of our experimental work a mil tungsten Whisker sharpened electrolytically with a tip diameter of less than 0.1 mil was used as one electrode or Whisker, the other electrical contact usually being made by soldering the alloy to a suitable conductor. However, tests have shown that the peak back voltages of rectiers made from the alloys of our invention are little affected by the metal of which the Whisker is made. Whiskers made of the following metals have been tried and only very slight deviations were noted over a large number of points of contact with the alloys of our invention: Mn, Pt, Ta, Ni, Fe, Zn, Mo, W, Au, Cu, Ag, Zr, Pt-Ir, and Pt-Ru. It appears therefore, that choice of a Whisker material may be determined on the basis of requirements other than the peak back voltage on rectifiers using the alloys. These electrodes or whiskers may have contact with the surfaces of the alloys as formed upon solidification, or on surfaces exposed by breaking the melt. As mentioned above, however, it is desirable to grind and etch the surface. Thus in one method of producing rectiers using the alloys of our invention, the melts, which usually were of pellet form 5 to 10 millimeters thick, may be cut into thin plates or slabs and a surface thereof ground with a suitable abrasive such as 6.00 mesh alumina (A1203). The abrasive used is not critical in that it has been found that other abrasives such as CrzOs, MgO, Va2O3, SnOz, ZnO and 4-0 paper are equally satisfactory. This may then be followed by a further grinding step with fine emery paper although this grinding step may be eliminated, if desired, without substantially altering the nal product. The surface of the plate or slab is then etched with a suitable etching solution which in one modification of our invention has the following approximate composition:

4 parts by volume hydroluoric acid (48% reagent) 4 parts by volume distilled water 2 parts by volume concentrated nitric acid l 200 milligrams Cu(NO3) 2 to each, 10 cc. of solution Such a solution will satisfactorily etch the surface of the plates or slabs in about 1 to 2 minutes at room temperature and may be applied with either a swab or by immersing the surface in the solution. This etching is not particularly critical but care should be taken not to unduly extend the etching since then a high polish is produced which may impair the performance of the alloy.

We have also found that other types of etches may be used effectively on the germanium alloys of our invention in addition to the etching above described. Modified etching solutions and procedures are as follows:

A solution consisting approximately of 1 gram stannyl chloride in 50 cc. of H2O may be used as an electrolytic bath for etching the alloy surfaces. Immersing the alloy as the anode in this solution will result in satisfactory etching within about 11/2 minutes at about 2% volts applied.

An alternative modification of an electrolytic etching solution may comprise 5 parts concentrated HNO; and 50 parts H2O by volume. Using the alloy as the anode for about 11/2 minutes at 1 to 2 volts will result in a satisfactory etch.

Reference may now be had to Figure 2 of the drawings illustrating the effect of etching of one of the alloys of our invention. The alloy selected to illustrate the effect of etching is identified as melt 24 P-OU136-.25Sn. This melt as appears from the aforesaid designation constitutes .25 atomic percent tin. The curve identified by reference numeral l illustrates the electrical characteristic of the germanium-tin alloy above identied in which the surface was ground with 600 A1203 but not etched. 'Ihe curve indicated by the reference numeral 2 illustrates-the electrical characteristics which were obtainedon a freshly broken surface of an alloy of the above composirst described.

The curve indicated by the reference numeral 4`il1ustrates electricalr characteristics of another point on the alloy after etching as described in connection with curve 3, the curves 3 and 4 representing the best and poorest performances, 'respectively, of the particular germanium tin alloy above identified, after etching( It is to be observed that in this graph the voltage scale in the forward direction is there expanded by a factor of 10 as compared to the voltage scale indicating the high back voltage characteristics of the alloys of our invention. As indicated, the currents are given in milliamperes.

It will be observedfrom an examination of Figure 2 that the electrical characteristics of a rectier using a broken surface exhibit high back voltages and forward conductances within the range of values obtained when using a ground and etched surface. However, such broken surfaces are shiny and geometrically irregular so that the Whisker tends to skid which is undesirable in assembling permanent rectier units. From Figure 2 it is apparent that the high back voltage and high back resistance properties are inherent in the alloys and that the etching is effective for restoring such properties after grinding. Further, we have discovered that natural'surfaces formed when solidifying the alloys in vacuum will, if not contaminated or otherwise affected by grinding, give high back voltages and high back resistances when mounted and tested in air.

For certain applications of these rectiers it is desirable that theyhave back resistances exceeding one megohm at about 5 volts. Using the procedure described above will occasionally produce suchhigh back resistances. However, we have found that a substantial and permanent increase in the back resistance can be effected by applying power overloads across the contact, for short intervals of time, each of length about 1A to l second or longer. The power treatment can be effected with the use of either alternating or direct current. By gradually increasing the voltage applied, and hence the current passed by the contact during successive pulses, an optimum valuecan be found to produce the maximum back resistance for a given contact. For direct current treatment in the forward direction such optimum' current values range from about 200 to 890 milliamperes. For alternating power treatment the optimum values of forward peak current range from about 300 milliamperes to 100) milliamperes. One can apply such alternating current treatment simply by connecting the rectier in series with a current limiting resistance and the secondary of a'transformer. Depending upon 'the size of this current limiting resistance, values of l to lil ohms have been used, voltage pulses ranging from '7 to 60 volts across the rectifier and-resistance serve to yield the maximum increase in back resistance.

Table III shows the permanent effects of4 such power treatment upon a few typical rectiers usingI alloys of our invention and prepared as described. It will be seen from the table that the most, significant effect of the power treatment is the increase in the back resistance as measured at about 4.5 volts. This resistance is increased byfaotors ranging from about 10 to 50 times the l0 values measured before treatment. Relatively minor increases of to 20 percent are effected on the peak back voltage. Forward currentsy at one voltare in general decreased by amounts `ranging. from l0 to 50 percent.

TABLE III EJjects of powe1` treatment (Values before power treatment are followed in parentheses by values after power treatment) Forward Peak Back Current at Back Rlsg' Alloy used inl Rectiiier alt-age one volt ance ts (vous) (mini- "0 h ampes) mego ms 6 (6) .'15 (8) 8 (8) 05 (l 8) 24 (10) 04 (7. 5) 20 (17) 48 i (l5) 40 (10) 20 (4) 10 (8) 20 (l) (10. 5) 13 (2) 18 (10) 10 (4) 4 (4) 40 (2) 10 (6) 20 (3) l0 (6. 4) '40 (7. 5) 15 (10) 20 v(3) 14 (8) 3S `(2v 5) 30 (15) 81 (3. 9) 29U`Nz 16 (10) l. 0 (7. 5)

It has been demonstrated above that the high back voltage, high back resistance, and good forward conductance properties disclosed are inherent in the germanium alloys of our invention. Modifications of surface treatments or power treatments'as described above will, however, vary the magnitude ofY these' properties within certain general' limits'. For example, on a given alloy surface, variations in surface treatment Vand power treatment may be expected to vary the average peak back voltage by a factor of about 2, the average forward current by a factor of about 2, and the average back .resistance by factors up to 50.A It Vwill be noted that the back resistance is the property mestv sensitive to variations in treatment, particularly to power treatment.

The following Table IV summarizes, on the basis of all melts made in experimental work conducted under our invention, the approximate figures of the minimum, average, and maximum values of peak back voltage and forward current at one volt whichmight be expected on the germanium alloys consisting of the addition of a single element.

TABLEr IV Peak Back Voltage Forward Current at One (volts) Volt (milliamperes) Alloy Min Ave Max Min Ave Max 25 75 150 2 15 30 8() 160 7 10 25 75 150 5 l5 25 25 75 150 5 l0 25 20 50 90 7 l5 30 25 50 100 5 l2 20 25 70 135 l 15 25 65 ll() 5 15 25 l0 50 l0() 2 10 20 2O 50 105 5 12 15 l5 50 125 7 13 2() l5 40 100 10 l5 3() 25 4.0 80 7 10 20 l0 30 70 3 7 15 20 30 35 l() 15 20 20 40 70 3 15 30 l5 4() 75 l 5 40 l5 25 40 5 15 40 l0 25 65 l0 25 40 20 25 50 2 5 20 It will appear from the above table that the ranges of values for the better alloys appear to be quite similar. Differences enter in the manner in which the values, within the ranges indicated, are concentrated. For example, the nitrogen alloys can usually be expected to have 70 to 90 percent of back peak voltages over 60 volts. Values on tin melts are more uniformly spread Within the range of the limit's given above. For the tin melts approximately 50% of the points on the surfaces thereof will have voltages above 60 volts. It appears that the pure germanium alloyed with tin or melted in an atmosphere of nitrogen represents the most advantageous alloy. Following them, alloys of pure germanium with calcium, strontium or nickel appear to be in order. It is to be understood, however, that one skilled in the art Working within the range of the alloys herein disclosed will readily be able to produce alloys having high back voltage and resistance characteristics and good forward conductances.

In Figure 3 of the drawings we have shown one type of rectifier in which our invention may be embodied. In the form of the device there shown a wafer 5 which may be of any of the germanium alloys above disclosed is mounted to have a low resistance non-rectifying contact with a metal electrode member 6. An electrode or Whisker 'l is connected at one end to an electrode supporting member 8 with the end of the Whisker in contact with the surface of the germanium alloy wafer 5. The standard 9 provides for mounting of the members supporting the Wafer 5 and electrode or whisker 1l in insulated relation. The rectifier contemplated by our invention may be of various forms, the only critical constructional feature being that the germanium alloy Wafer comprising the semiconductor, and the whisker for contacting the surface of the wafer being arranged and supported so that one end of the Whisker engages the semi-conductor surface. It is understood that suitable leads are connected to the wafer or semi-conductor and to the Whisker or metal electrode so that the device may have application in any desired circuit for use in the rectication of current.

While we have disclosed what we consider to be the preferred embodiments of our invention, it will be understood that various modifications may be made therein without departing from the spirit and scope of our invention.

We claim as our invention:

1. An electrical device comprising a semi-conductor, a counter electrode having substantially point contact with said semi-conductor and a second electrode having an area of contact with said semi-conductor which is large compared to that of the counter electrode, said semi-conductor consisting of germanium of the order of 99% purity in combination with at least one of the elements from the group consisting of vanadium, columbium, tantalum, and bismuth, said device having a peak back voltage in the range in excess of 10 volts and approaching the order of 200 volts.

2. An electrical device comprising an alloy formed of a mixture of germanium having a purity of the order of 99% and vanadium in an amount of between 0.15 and 1.0 atomic percent, and a pair of electrode elements in contact with said formed alloy, one of said electrode elements having substantially point Contact with said alloy and the second of said electrodes having an 12 area of contact which is large compared to that of the point contact electrode.

3. An electrical device comprising an alloy formed of a mixture of germanium having a purity of the order of 99% and columbium in an amount of 0.5 atomic percent, and a pair of electrode elements in contact with said formed alloy, one of said electrode elements having substantially point contact with said alloy and the second of said electrodes having an area of contact which is large compared to that of the point contact electrode.

4. An electrical device comprising an alloy formed of a mixture of germanium having a purity of the order of 99% and tantalum in an amount of 0.44 atomic percent, and a pair of electrode elements in contact with said formed alloy, one of said electrode elements having substantially point contact with said alloy and the second of said electrodes having an area of contact which is large compared to that of the point contact electrode.

5. The method of making an electrical device which comprises mixing germanium having a purity of the order of 99% with at least one of the elements from the class consisting of vanadium, columbium, tantalum, and bismuth, applying heat to the mixture to reduce the mixture to a fluid state, maintaining the heat for a time period sufficiently long to permit mixing of the selected constituents, removing the heat to permit solidication of the mixture, cutting from the ingot formed upon mass solidification wafers to which contact electrodes may be applied, and applying contact electrodes to said wafers.

6. The method of claim 5 including the additional steps of grinding the severed wafers and then etching the ground surfaces to provide optimum contact points for said contacting electrode members.

7. The method of claim 5 including, in addition, the step of etching a surface of a wafer cut from the ingot before applying a contact electrode.

8. The method of claim 5 including, in addition, the step of etching a surface of a wafer cut from the ingot in a solution made up of approximately 4 parts by volume of hydrouoric acid (48% reagent), 4 parts by volume of distilled water, 2 parts by volume of concentrated nitric acid, and 200 milligrams Cu(NO3) 2 to each l0 cc. of solution for a time period in the general range of between 1 and 2 minutes.

9. The method of claim 5 including securing one of said electrodes to one surface of a cut wafer, locating a second substantially point contact electrode upon a different surface of the cut Wafer and in substantially point contact therewith, and then applying electric power between the electrodes and the wafer.

10. The method of claim 9 including, in addition, regulating the supplied current in the forward direction through the cut wafer and limiting the current value to the range between 200 and 800 milliamperes applied in pulses of between 1A to 1 second in length.

l1. The method of claim 10 including, in addition, the steps of connecting the formed device in series with a current limiting resistance and a secondary of a transformer of alternating electric currents controlling the peak current in the forward direction to the order of between 300 and 1000 milliamperes so that the voltage across the device and limiting resisttion.

ance is of the order of between 7 and 60 volts and the limiting resistance is of the order of 10 to 40 ohms and regulating the period of application of the alternating current to intervals varying between 1/4 and 1 second in time devia- 12. The method of claim 5 including electrolytically etching one of said wafers as an anode in a solution in the proportions of approximately 1 gram stannyl chloride to 50 cc. water for about 11/2 minutes at about 21/2 volts.

13. The method of claim 5 including electrolytically etching one of said wafers as an anode in a solution in the proportions of approximately 5 parts concentrated nitric acid to 50 parts Water by volume for about 11/2 minutes at about 1 to 2 volts.

14. An electrical device comprising a semiconductor, a counter electrode having substantially point contact with said semi-conductor and a second electrode having an area of contact with said semi-conductor which is large compared to that of the counter electrode, said semi-conductor consisting of germanium of the order of 99% purity in combination with at least one of the elements from the class consisting of vanadium, columbium, tantalum, and bismuth, said device having a peak back voltage in the range in excess of 10 volts and approaching the order of 200 volts, the back resistance of said device being in the order of between 10,000 ohms to several megohms at about 5 volts and the forward current being in the range of between 5 and 40 milliamperes at one volt in the low resistance direction of current iiow through the device.

KARL LARK-HOROVITZ.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,530,110 Woodyard Nov. 14, 1950 

1. AN ELECTRICAL DEVICE COMPRISING A SEMI-CONDUCTOR, A COUNTER ELECTRODE HAVING SUBSTANTIALLY POINT CONTACT WITH SAID SEMI-CONDUCTOR AND A SECOND ELECTRODE HAVING AN AREA OF CONTACT WITH SAID SEMI-CONDUCTOR WHICH IS LARGE COMPARED TO THAT OF THE COUNTER ELECTRODE, SAID SEMI-CONDUCTOR CONSISTING OF GERMANIUM OF THE ORDER OF 99% PURITY IN COMBINATION WITH AT LEAST ONE OF THE ELEMENTS FROM THE GROUP CONSISTING OF YANADIUM, COLUMBIUM, TANTALUM, AND BISBUTH, SAID DEVICE HAVING A PEAK BACK VOLTAGE IN THE RANGE IN EXCESS OF 10 VOLTS AND APPROCHING THE ORDER OF 200 VOLTS. 